Generally, a sound that is heard by human beings is a wave. The sound wave that is transmitted to the ears of the human beings vibrates the air molecules within the ears. The movement of the molecules, in turn, vibrates the tympanic membrane, that is, eardrum, so that the human beings can hear the sound.
The sound wave entering the external ears of the human beings is transmitted to the tympanic membrane. The vibration of the tympanic membrane generated by the sound wave is transmitted to the cells of the internal ears, so that the human beings can hear the sound.
The audible frequency in which the human beings can hear the sound is within the range of 20 Hz to 20,000 Hz. The best audible frequency range, that is, so called ‘communication voice range’ is 125 Hz to 8,000 Hz. Commonly, the human beings use the range of 500 Hz to 2,000 Hz for their communication.
Most of the human beings can hear the sound in the range of 20 Hz to 20,000 Hz. However, some of the human beings having auditory disorders such as disorders of external ears or middle ears, presbycusis which is a hearing loss that gradually occurs, as they grow older, and hearing loss as an occupational disease cannot hear the sound in the only specific frequency range or all over the frequency range.
A bone conduction telephone and a hearing aid vibrator by using a piezoelectric device which is used for such people having hearing loss are disclosed in the Korean Utility Model Registration No. 221534.
The bone conduction telephone and the hearing aid vibrator by using the piezoelectric device according to the utility model in the Korean Utility Model Registration No. 221534 comprises the piezoelectric device 100 which generates a physical vibration by an electric voice signal being applied, vibrating plates 110 disposed at both end side of the piezoelectric device 100 to apply the electrical signal to the piezoelectric device 100 through a conducting wire 130, an electrode plate 120, a front contacting portion 140 connected to the electrode plate 120 to transmit the vibrating to the exterior, and a cover 150 which supports and protects the piezoelectric device 100. In the utility model, the piezoelectric device 100 generates a vibration with piezoelectric effect. The generated vibration has a voice frequency corresponding to the electrical voice signal which is applied through the vibrating plates 110 and the electrode plate 120. The generated vibration is transmitted to the exterior as a vibration signal.
However, bone conduction telephone and the hearing aid vibrator by using the piezoelectric device according to the utility model are too theoretical to be implemented. The utility model has so many problems for its commercial implementation. Since the piezoelectric device has high impedance and high input voltage, the output sound pressure is too low and the miniaturization of the products is too difficult.
In order to solve the aforementioned problems, a bone conduction hearing aid apparatus and a bone conduction telephone have been proposed and developed to directly vibrate skull bone and stimulate the auditory nerve for hearing the sound.
The bone conduction speaker which directly vibrates the skull bone and stimulates the auditory nerve for hearing the sound is disclosed in the Korean Patent Registration No. 310891 which is registered on Sep. 21, 2001.
The bone conduction speaker disclosed in the Korean Patent Registration No. 310891 comprises a frame having a side wall and a lower wall to define a certain space; a protector of which an inner side peripheral portion is combined to an upper surface of the side wall and of which a center has an opening; a damper for reducing intensities of vibration and sound by performing stabilized vibration, wherein the damper is disposed between the upper end of the side wall and the lower surface of the inner side peripheral portion; a main body for supporting the inner side end of the damper, wherein the main body is disposed within the space; an extending portion which is incorporated into the main body and extended downwardly; an yoke for focusing magnetic flux; a magnet which is disposed at the interior of the extending portion and of which upper surface is closely contacted to the lower surface of the main body; an upper plate which is disposed at the interior of the extending portion to form a magnetic circuit together with the yoke and the magnet; a vibration contacting portion which is externally exposed through the opening to be directly contacted to the head of human beings; a fixing means for fixing the protector, the yoke, the magnet, and the upper plate as one body; and a voice coil which is disposed in the clearance between the upper plate, both ends of the magnet, and the inner surface of the extending portion and of which a lower end is fixed to the lower wall.
However, the conventional bone conduction speaker has a problem that high driving voltage and high power consumption are necessary to vibrate the yoke, the magnet, and the upper plate since the yoke, the magnet, and the upper plate serve as parts for generating the magnetic field as well as parts of vibrating upwardly and downwardly.
In addition, in the conventional bone conduction speaker, high strength of the magnetic field is necessary and large sizes of the yoke, the magnet, and the upper plate which are above a specific size are necessary to vibrate the yoke, the magnet, and the upper plate since the yoke, the magnet, and the upper plate serve as parts for generating the magnetic field as well as parts of vibrating upwardly and downwardly.
Therefore, the conventional bone conduction speaker has another problem that it is difficult to be miniaturized and employed to a subminiature mobile phone, a subminiature headset, a hearing aid apparatus, and so on.
Furthermore, in the aforementioned bone conduction speaker, the voice coil is fixed on the base plate and the mastoid, the yoke, the magnet, and the upper plate are vibrated upwardly and downwardly by the magnetic field which is generated by the voice coil. Therefore, the conventional bone conduction speaker has still another problem that it is difficult to transmit the vibration of the mastoid to the user accurately since the mastoid which is contacted to the bone skull of the user is difficult to vibrate upwardly and downwardly due to heavy weights of the mastoid, the yoke, the magnet, and the upper plate.
In addition, the conventional bone conduction speaker has further still another problems that it may be easily out of order due to any infinitesimal external impacts and its reliability may be being deteriorated since the internal parts such as the yoke and the magnet for generating the magnetic field are disposed at the upper portion of the bone conduction speaker.
In order to solve the aforementioned problems, the applicant of the present invention filed a dual application of a patent application (Korean Patent Application No. 10-2002-0048318) and a utility model application (Korean Utility Model Application No.20-2002-0024417), tiled “a subminiature bone conduction speaker by using a vibrating plate and a mobile phone having the same,” on Aug. 16, 2002 with the Korean Intellectual Property Office.
Although the aforementioned subminiature bone conduction speaker by using a vibrating plate has advantages that it is possible to reduce a driving voltage for reducing power consumption, to easily facilitate the miniaturization of the bone conduction speaker, to accurately transmit the signal sound to a user, and to have good resistance against an external impact, it has a problem that the user cannot hear sounds selectively.
Therefore, development of an advanced bone conduction speaker having a function of a subminiature bone conduction speaker together with a function of a general speaker for hearing sounds has been greatly needed.